The use of high velocity, abrasive-laden liquid jets to precisely cut a variety of materials is well known. Briefly, a high velocity waterjet is first formed by compressing the liquid to an operating pressure of 35,000 to 70,000 psi, and forcing the compressed liquid through an orifice having a diameter approximating that of a human hair; namely, 0.001-0.015 inches. The resulting highly coherent jet is discharged from the orifice at a velocity which approaches or exceeds the speed of sound.
The liquid most frequently used to form the jet is water, and the high velocity jet described hereinafter may accordingly be identified as a waterjet. Those skilled in the art will recognize, however, that numerous other liquids can be used without departing from the scope of the invention, and the recitation of the jet as comprising water should not be interpreted as a limitation.
To produce the abrasive-laden waterjet, the high velocity jet thus formed is passed through a mixing region, which is typically within the same housing as the aforedescribed components. A quantity of abrasive is entrained into the jet in the mixing region by the low pressure region which surrounds the flowing liquid in accordance with the Bernoulli Principle. The abrasive is typically (but not limited to) a fine silica or garnet, and is coupled into the mixing region from a hopper which is external to the nozzle housing.
The abrasive-laden waterjet is discharged against a workpiece which is supported closely adjacent to the discharge
information and details end of the nozzle housing. Additional concerning abrasivejet technology may be found in my U.S. Pat. No. 4,648,215, the contents of which are hereby incorporated by reference. The term "abrasivejet" is used herein as a shorthand expression for "abrasive-laden waterjet" in accordance with standard terminology in the art.
Although abrasivejets have been used to cut a wide variety of materials, no commercially satisfactory apparatus has been available for drilling brittle, composite, or laminated materials. These materials tend to chip, crack, fracture, or delaminate when impinged upon by the jet. One presently known technique for cutting glass is disclosed in U.S. Pat. No. 4,072,042, wherein a starting hole is first drilled through the workpiece by a relatively low-pressure abrasivejet, and the pressure of the jet-forming fluid is then increased to the high pressure required for cutting.
The Bernoulli effect at such low pressure operations appears to be insufficient to properly entrain abrasives from the external hopper, and cutting systems utilizing low-pressure drilling accordingly provide inconsistent results. It has been found, for example, that the drilling rates are sometimes lower than expected and, in many cases, only limited drilling depths are possible. These drawbacks are aggravated when the starting hole is drilled at a point relatively remote from the workpiece edge and the portion of the workpiece containing the drilled starting hole must usually be scrapped because of damage to the area adjacent the hole.